As the semiconductor industry progresses into nanometer technology in pursuit of higher device density, higher performance, and lower costs, stricter demands have been placed on the lithography tools used in semiconductor manufacturing. Techniques such as extreme ultraviolet (EUV) lithography have been utilized to support critical dimension (CD) requirements of smaller IC devices. EUV lithography uses radiation in the EUV region, having a wavelength of about 1-100 nm, such as 13.5 nm, which is much shorter than the wavelengths in the deep ultraviolet (DUV) lithography (e.g., 193 nm lithography). EUV lithography uses masks (or reticles) that reflect EUV radiation from a radiation source towards a target (such as a silicon wafer), thereby transferring patterns from the masks to the target. Any defect on the surface of a EUV mask (as well as defects embedded in the EUV mask) may cause imaging defects on the target. Therefore, it is important to protect the EUV mask surface during lithography processes.
Unlike the masks used in DUV lithography, which traditionally employ a pellicle to protect the mask surface, it is currently difficult to manufacture an effective pellicle for EUV masks on a large scale. One reason is that the wavelength of the EUV radiation is very short and membranes of traditional pellicles would absorb the EUV radiation so much that they would deform due to excessive heat after few uses and also would substantially reduce the EUV energy reaching the target. Improvements in these areas are desired.